One large disadvantage of conventional NMR Spectrometers is that the detection system is based on Faraday's law. That is, the receiver coil only sees the rate of change of magnetic flux, this being larger at larger frequencies. Larger frequencies are achieved by immersion of the sample in very strong magnetic fields. As more capital is ploughed into the research and development of more powerful superconducting magnets, a few scientists have considered alternative detectors. These use the direct current (dc) superconducting quantum interference device (SQUID), details for which are (1) Claude Hilbert & John Clarke Appl. Phys. Lett. 43 694 (1983), and (2) Claude Hilbert John Clarke, Appl. Phys. Lett., 45 799 (1984). The dc SQUID detector is an alternative as it detects the magnitude of flux and is, therefore, independent of frequency and magnetic field strength.
Dc SQUID'S have been applied to a low noise radiofrequency amplifier (see U.S. Pat. No. 4,585,999).
The typical dc SQUID is a small device. However, conventional dc SQUID'S operate at liquid Helium temperatures (4.2 K.). To construct a NMR or NQR Spectrometer requires a helium cryostat in which to hold the SQUID and detection systems. In order to achieve adequate shielding and supply of liquid helium, this cryostat is quite large and not portable. Furthermore, to achieve further shielding from stray radio frequency fields, the SQUID detector has to be placed within a faraday cage.
Even though the dc SQUID Spectrometer is essentially more simple than the conventional NMR Spectrometer, this advantage is lost as the space required for the instrument is still large. The main drawbacks are, therefore, the dc SQUID operating temperature and the required r.f. shielding.
Recently, many new superconducting materials have been discovered based on the mixed oxides of a Lanthanide, copper and barium. These material are superconducting at liquid nitrogen temperatures (77 K.).